Support for stateful modifiers like controlled filters.
This is similar to Synthesizer.Causal.Process
but we cannot replace the Modifier structure by the Causal structure
because the Modifier structure exhibits the state
which allows stacking of modifiers
using an efficient storage for the stacked state.
More precisely, because Modifiers exhibits the type of the state,
we can ensure that the state type of several modifiers is equal
and thus the individual states can be stored in an array or a StorableVector.